Mast cells (MCs) are important in allergic diseases, tissue injury, and protection from infections. MC numbers and their effector functions are dynamically regulated by constituents of the tissue microenvironment. This application for continued support focuses on the role of cysteinyl leukotrienes (cys-LTs) and their receptors in the regulation of MC function. During the last funding period, we recognized that human MCs (hMCs) expressed both of the known receptors for cys-LTs (CysLT1 and CysLT2 receptors), and defined different functions for each. We identified specific modulatory functions for IL-4 in regulating cys-LT-dependent signaling events as well as CysLT2 receptor expression, and defined differential and complementary functions for the two cys-LT receptors by developing null strains of mice. Our preliminary studies now demonstrate that cys-LTs play an unanticipated, essential role in mediating reactive mastocytosis in a model of allergen-induced pulmonary inflammation. Both leukotriene C4 synthase-null mice (which cannot generate cys-LTs) and CysLT1 receptor-null mice completely lack MCs in the inflamed epithelial surfaces of the bronchi. Moreover, we have demonstrated that both cys-LTs and a nucleotide, UDP, behave as co-mitogenic growth factors for hMCs in vitro, acting by a pathway that requires CysLT1 receptors, Gi proteins, and the ERK MAP kinase cascade. Finally, we demonstrate that CysLT1 receptors on MCs constitutively form heterodimers with CysLT2 receptors and with UDP- selective P2Y6 receptors, respectively, and that each heterodimer differentially regulates the function of the therapeutically relevant CysLT1 receptor. The Specific Aims are: 1. To determine the molecular basis and functional consequences of dimerization of P2Y6 and CysLT2 receptors with the CysLT1 receptor, and; 2. To define the mechanisms through which P2Y6/CysLT1 receptors amplify stem cell factor (SCF)-dependent growth of hMCs, and mediate reactive mastocytosis in vivo using newly developed null mouse strains. Project Narrative: This proposal seeks to understand how a class of chemicals called cysteinyl leukotrienes control the function of mast cells, a type of immune cell. Since both mast cells and cysteinyl leukotrienes are important in asthma and are also believed to be important in heart disease and other conditions, these studies are anticipated to reveal potential new treatments for these common diseases.